1. Field of Invention
This invention is directed to the field of growing bulk semiconductor crystals from melts. More particularly, this invention is directed to a Czochralski crystal growing system for growing single crystals of semiconductor materials and also to methods of adding dopants to semiconductor materials and growing single crystals using the crystal growing system.
2. Description of Related Art
The Czochralski crystal growing method is commonly used to grow bulk semiconductor single crystals from melts of semiconductor materials contained in crucibles of crystal growing furnaces. In this method, a seed crystal is brought into contact with the melt and then slowly withdrawn to grow a single crystal.
Extrinsic semiconductor wafers include a controlled amount of dopant material to produce desired electrical properties. An effective amount of dopant to achieve desired electrical properties can be added to a cold charge of polycrystalline semiconductor material, typically stored in a sealed environment in a clean room. It is necessary to break the seal in order to add dopant to the cold charge. Breaking the seal can potentially expose the semiconductor material to contamination.
After the dopant is added to the cold charge, these materials are melted in a crucible in a crystal growing furnace to form a molten mixture. As stated, a seed crystal is used to pull a single crystal ingot from the molten mixture. During melting, dopants having a high vapor pressure such as antimony tend to evaporate from the melt and cause the dopant concentration in the melt to change. As a result, the concentration of such dopants in the ingots can fall outside of the specified range to produce single crystals having desired electrical properties.
It is also known to add dopants to melts of semiconductor materials at the crucible. The dopants are typically granules or flakes. The melt is maintained in an inert gas atmosphere to avoid contamination. During doping of the melt, this gas flow can cause the dopant to be scattered in the furnace near to the crucible. Consequently, a portion of the specified amount of the charge is not added to the melt. The scattered dopant can deposit on protuberances in the furnace. If the amount of dopant that is actually added into the melt is below the specified amount, the dopant concentration in the melt cannot achieve the desired electrical properties in single crystals grown from the melt. In addition, dopants that deposit near the crucible can fall into subsequent melts in the crucible and cause such melts to have dopant concentrations above the specified level. These dopants can also introduce contaminants into the melt. Furthermore, this method requires the pull chamber of the crystal growing furnace to be opened and evacuated several times, which can allow contamination of the melt. Thus, this method occasionally is unable to add desired amounts of dopants to melts. Uncontrolled fluctuations in the dopant concentration in the melt can result in grown single crystals that are unsuitable for use in electronic devices that require precise dopant levels. Such single crystals typically must be scrapped, which represents a significant expense to manufacturers.
It is further known to attach solid dopants directly to seed crystals and to lower the seed and dopant into the melt so as to melt the dopant, as disclosed, for example, in U.S. Pat. No. 5,406,905 and Japanese patent application J 62-153188. Although this method overcomes the problem of scattering of the dopant by gas flows within the furnace, the dopant may drop prematurely into the melt, causing premature doping and also splashing of the melt. Premature doping is especially important for dopants having a high vapor pressure, which need to be added to the melt at the appropriate time to reduce evaporation. In addition, the dopant is not prevented from being exposed to contamination during handling outside of clean environments.
Another important concern during single crystal growing by the Czochralski method is reducing the generation of dislocations when the seed crystal is dipped into the melt. Dislocations are generated at the interface between the seed crystal and the single crystal due to thermal shock caused by the large temperature differential between the seed crystal and relatively much hotter melt. Known methods deliberately form a narrow neck portion in the single crystal just below the seed crystal, so that dislocations in the seed crystal will not propagate into the bulk of the single crystal. This approach does not, however, satisfactorily reduce the generation of dislocations nucleated in the single crystal by thermal strains.
Thus, there is a need for an improved Czochralski crystal growing system that overcomes the above-described disadvantages of known delivery systems. More particularly, there is a need for a Czochralski crystal growing system that can prevent the contamination of dopants prior to being added to melts, protect dopants from heat and gas flows in crystal growing furnaces, and can be used to consistently add specified amounts of dopants directly into melts. There is also a need for a Czochralski crystal growing system that can reduce the generation of dislocations in single crystals due to thermal strains.